A senior fire consultant who worked on the Grenfell Tower refurbishment has conceded his “ad hoc advice” could have been interpreted as approving the use of combustible insulation on the project, the inquiry into the disaster has heard.

Terry Ashton, of fire engineering firm Exova, was provided architectural drawings of the planned original zinc cladding in September 2014 and was asked for his advice about the use of fire-stopping measures in the system.

However, by this time the new contractor Rydon had not kept Exova on as a “fully paid-up member of the design team” and Mr Ashton said his advice was given on an “ad hoc” basis, adding he did not feel obliged to “give these drawings much attention”.

He replied to project architects Studio E: “If the insulation in the cavities behind the rainscreen cladding is combustible you will need to provide cavity barrier …” to stop fire spreading from one flat to the one above.

National building guidelines under the widely used Approved Document B (ADB) specified that materials had to be of “limited combustibility”.

Combustible materials were deemed acceptable only if they were approved in separate tests as part of an overall system, he told the inquiry.

Mr Ashton said he offered his advice despite not knowing what kind of insulation was being proposed and without knowing if the architects even planned on using the tested method.

However, the same day he had been sent an email with an attachment specifying the designer’s intention to use CelotexRs5000 insulation, a combustible material.

Mr Ashton failed to open the attachment before offering his advice, and he denied inquiry lawyer lawyer Kate Grange QC’s suggestion it was “patently clear that you needed to open that attachment and then respond”.

Ms Grange said: “Would you agree now looking at this email that it appears to suggest combustible insulation is permissible as long as there are cavity barriers?

He replied: “It could be interpreted that way, yes I agree.

“I didn’t know what insulation they were using.

“I didn’t know whether they were going compliance with ADB or using a system which had been tested.”

Ms Grange said: “Given that you didn’t know it wasn’t the correct advice to give was it?

“To suggest if insulation is combustible it’s permissible as long as there are cavity barriers.”

Mr Ashton said: “I could have worded it better, I concede that.”

He added it was “not my job at that stage of the project” to advise Studio E about the risks of combustible insulation, adding: “I think designers should have been aware themselves.”

He went on: “If this issue had come up at a time when I was a fully paid-up member of the design team then all of these things would be discussed in much more detail.

“I’m being asked to comment on things when we have effectively left the design team and only in little bites of information not on the full picture.

“I didn’t give these drawings much attention.

“I just wanted to know in broad terms what they were doing.

“I wasn’t asking for full construction details for me to spend hours and hours of work for which I wasn’t entitled to be paid.”